Electrical cable having an outer sheath with improved pyrolysis properties

ABSTRACT

An electrical cable jacket employs chloro-sulfonated polyethylene and chlorinated polyethylene elastomeric polymers, magnesium oxide, and extenders and vulcanizing constituents. The jacket composition will not sustain combustion and, moreover, does not emit substantial smoke products during pyrolysis.

United States Patent [191 Alia [ ELECTRICAL CABLE HAVING AN OUTER SHEATH WITH IMPROVED PYROLYSIS PROPERTIES [75] Inventor: Dominic Anthony Alia, Oakland,

[73] Assignee: The Okonite Company, Ramsey,

[22] Filed: Dec. 20, 1972 [21] App]. No.: 316,848

[52] US. Cl 260/23 R, 260/41 A, 260/897 C [51] Int. Cl. .Q C08f 29/12 [58] Field of Search 260/23 H, 897 C, 41 A v [56] References Cited UNITED STATES PATENTS 3,405,204 10/1968 McCormack ..260/23H June 28, 1974 3,600,469 55/1971 Sato etal ..260/23'H Primary Examiner-M. J. Welsh Attorney, Agent, or FirmStephen B. .ludlowe [57] ABSTRACT 2 Claims, No Drawings 1 ELECTRICAL CABLE HAVING AN OUTER SHEATH WITH IMPROVED PYRoLYsIs PROPERTIES DISCLOSURE OF INVENTION This invention relates to electrical wire and cable and, more specifically, to an improved insulated, jacketed cable which will not support combustion and, moreover, which does not emit substantial smoke during pyrolysis.

It is obviously important in numerous applications of present day interest that electrical cable, comprising one or more mutually insulated electrical conductors surrounded by an outer jacket, or sheath, not sustain combustion. Such cable does therefore not serve as fuel to aid a local fire, or as a medium to proliferate any such combustion.

However, in certain environments, it is highly desirable also that the cable jacket not emit substantial smoke during burning. Thus, for example, local trash fires in close proximity to electrical power and signaling cables are not uncommon in subway systems or the like, these fires being disruptive but not overly dangerous per se. However, the psychological and, to a lesser extent, physiological effects of acrid smelling, black enshrouding smoke, produced by burning cable, on those confined in the environment, e. g., a stationary subway car, is indeed dangerous and has in fact caused injury and fatality. Other environments where smokeless cable is a patent desideratum are mining, construction (especially schools, hospitals and the like), shipbuilding and power (particularly for nuclear reactor control and shutdown).

However, it has long been appreciated as a general matter that those organic materials (as cable jacket materials typically are) which do not burn, smoke;and, conversely, those which dont smoke, burn. Thus, for example, conventional cable jacketings made of polychloroprene, chlorosulfonated polyethylene, polyvinyl chloride or chlorinated polyethylene, will not sustain combustion, but do yield very substantial smoke andoff-gas pyrolysis products when burned. This is typically the case for chlorinated elastomeric polymers.

Accordingly, economically realistic electrical cabling which would neither burn nor smoke has heretofore not been available.

It is therefore an object of this invention to provide an essentially smokeless electrical cable jacket material.

More specifically, it is an object of the present invention to provide such a smokeless cable jacket which may be readily fabricated in commercial quantities of the customary kinds of materials heretofore em- I ployed for electrical cable manufacture.

The above and other objects of the present invention are realized in a specific, illustrative embodiment thereof, wherein an electrical cable comprises at least one electrical conductor surrounded by an insulation layer, and an outer jacketing sheath. The cable jacket is preferably formed of chlor'o-sulfonated polyethylene and chlorinated polyethylene elastomeric polymers (aggregating approximately 50 percent by weight);

magnesium oxide (approximately 5 percent by weight); a tri-cresyl phosphate plasticizer (approximately 7.5 percent by weight); and additional extenders, vulcaniz- 2- ing agents, and processing aids as more fully set forth below.

The cable jacket formed as above-described is superior to comparable cable sheaths of the prior art. First, the jacket will not support combustion. Indeed, when exposed to direct flame, the operating time to cable electrical failure is several times longer for cables with the instant jacket than for polychloroprene or other conventional chlorinated polymer sheathed cables. Then also, cable in accordance with the present invention emitted very little smoke during pyrolysis vis-a-vis prior art cable.

The above-considered features and advantages of the present invention will become more clear from the following detailed discussion of specific embodiments and examples thereof.

In accordance with the basic principles of the present invention, a cable jacket system includes chlorosulfonated polyethylene (e.g., DuPonts Hypalon), and magnesium oxide. The system further comprises additional rubber-like polymer, extenders, a fillerpigmentation, vulcanizing agents all as set forth in particular detail hereinbelow.

The cable jacket formed by this invention will not sustain combustion and, in fact, resists an applied flame and continues to perform electrically for a period greatly exceeding that for a polychloroprene or other conventional chlorinated polymer jacketed cables.

Moreover, the cable jacket in accordance with the present invention off-gasses only minimal smoke when flame is applied thereto and that notwithstanding that the primary constituents thereof, chlorinated rubber, and most others as well, smoke when burned. The specific reasons why the instant cable jacket is smokeless are not precisely understood. However, by way of illustration and without limitation, it is postulated that an intumescence mechanism characterizing the instant cable is responsible for the smoke-suppressing and flame retardant cable properties. That is, when flame is applied to the cable jacket composition, it swells into a foamed or sponge-like physical texture. The expanded jacket matrix upon intumescence traps particulate matter (e.g., carbon black) which would otherwise escape upon pyrolysis to cause an acrid, opaque smoke. The multi-celled structure formed upon swelling also comprises relatively poor heat conductors, hence expanding the operative cable life under flame during which the jacket protects the interior insulation and electrical conductors from the heat source.

With the above considerations in mind, specific examples for practice of the present invention will now be considered.

EXAMPLE 1 An electrical cable jacketing material was formed of:

a. chloro-sulfonated polyethylene (CSP) elastomeric polymer, such as that vended under the trade name Hypalon by the DuPont company 40 percent by weight, as the basic rubber-like polymer matrix;

b. chlorinated polyethylene (CP) elastomeric polymer 10 percent by weight; for its mechanical elasticity;

c.'clay, principally comprising hydrated aluminum silicate 23.75 percent by weight. The clay is employed as a processing agent and extender for the rubber matrix, and to also impart processing elasticity to the basic rubber material;

d. carbon black as a filler and coloration agent 10 percent by weight;

e. magnesium oxide, 5 percent by weight. The magnesium oxide acts as a vulcanizing aid and, moreover, is understood to play a significant role in imparting the non-smoking, intumescing property to the final rubber jacket composition. The magnesium oxide may be utilized in a dispersion thereof protected from water attack as vended by Wyrough & Loser, Trenton; N.J.;

f. stearic acid 0.5 percent by weight, as a mill release processing aid;

g. tri-cresyl phosphate (TCP), a plasticizer with extremely low volatility (a boiling point at 420 C) 7.5 percent by weight;

h. paraffin, a processing aid and lubricant 2.0 percent by weight;

i. vulcanizing agents comprising sulfur (0.25 percent by weight) and tetramethylthiuram-disulfide (TMTD) 1.0 percent by weight.

The jacket material is formed by mixing the CSP, CP, clay percent of the 23.75 percent), carbon black, and magnesium oxide in an internal mixer (e.g., a Banbury) at a low speed (approximately 22 RPM), with the ram pressure applied, for approximately 3 minutes. The ram was then raised, and the stearic acid, TCP, paraffin, and remaining clay added. The ram was lowered and the constituents again mixed at low speed for approximately 2 minutes.

Finally, the sulfur and TMT D were added, and the composite system mixed for 1.5 minutes, maintaining the mixing temperature below 250 F by a flow of cooling water through the Banbury blades and shell. The composite material so formed was finally formed into cable jacketing by conventional milling and extrusion about the cable electrical conductors and surrounding insulation.

The above cable jacket system, which comprises a preferred embodiment of my invention, was exposed to a gas flame in a test chamber, and the same flame exposure applied to standard polychloroprene or other conventional chlorinated polymer jacketed cables. As above described, the cabling of the instant invention survived approximately two times longer before an electrical failure resulted vis-a-vis the polychloroprene or other conventional chlorinated polymer jackets. Moreover, while the cable of theinstant invention was being burned, only a de minimus amount of smoke emanated therefrom compared with the polychloroprene or other conventional chlorinated polymer jackets. During burning, the instant cable jacket intumesced.

FURTHER EXAMPLES Cable jacketing material was prepared as abovedescribed, but with the magnesium oxide present at the 2.5 percent and 7.5 percent levels. At 2.5 percent of Then also, the electrical cable jacket of Example 1 was fabricated, but: (1) in one case, using chlorinated paraffin rather than tri-cresyl phosphate as a plasticizer; and (2) in another example,'a calcined rather than hydrated clay. In niether case was the smokeless properties of the jacketing material so formed impaired.

In addition to the fire-exposure tests described above for the fully formed electrical cable including the sheaths of Example I, each cable jacket described herein was exposed to a like flame in a hood, and the optical impedance between a light source and a photodetector spaced therefrom measured as a function of time (as was a prior art polychloroprene or other conventional chlorosulfonated polyethylene, chlorinated polyethylene or polyvinyl chloride sheath material). In each case, the smoke reduction or substantial suppression far exceeded the properties of the above mentioned conventional materials. v

The above described arrangements and compositions are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention.

What is claimed is:

1. A composition suitable for electrical cable jacketing and disposed to enclose at least one conductor and insulation thereabout, said composition being essentially smokeless and consisting essentially of approximately 40 percent by weight chloro-sulfonated polyethylene, approximately 10 percent by weight chlorinated polyethylene, approximately 23.75 percent by weight clay, approximately 10 percent by weight carbon black, approximately 5 percent by weight magnesium oxide, approximately 0.5 percent by weight stearic acid, approximately 7.5 percent by weight tri-cres yl phosphate, approximately 2 percent by weight paraffin, approximately 0.25 percent by weight sulfur and approximately 1.0 percent by weight tetramethylthiuramdisulfide.

2. A composition suitable for electrical cable jacketing and disposed to enclose at least one conductor and insulation thereabout, said composition being essentially smokeless and consisting essentially of chlorosulfonated polyethylene and chlorinated polyethylene having uniformly dispersed therethrough at least about 2.5 percent by weight of magnesium oxide effective to impart an intumescent property to said composition when subjected to elevated temperatures. 

2. A composition suitable for electrical cable jacketing and disposed to enclose at least one conductor and insulation thereabout, said composition being essentially smokeless and consisting essentially of chloro-sulfonated polyethylene and chlorinated polyethylene having uniformly dispersed therethrough at least about 2.5 percent by weight of magnesium oxide effective to impart an intumescent property to said composition when subjected to elevated temperatures. 